In the oil and gas exploration and production industry, subsurface hydrocarbon-bearing formations are accessed by drilling bores from surface. In a typical drilling operation a drill bit is mounted on the lower end of a tubular string of pipe extending from surface. Drilling fluid or “mud” is pumped down the drill pipe string from surface and exits through jetting nozzles in the drill bit. The drilling fluid serves a number of purposes, one being to carry drill cuttings out of the bore, that is the drilling fluid entrains the cuttings as the fluid flows back up to surface through the annulus between the drill pipe and the bore wall. On surface the cuttings are separated from the fluid, such that the drilling fluid may be reused or recycled.
The drilling fluid may also be used as a medium to transmit information to surface. In particular, measurement-while-drilling (MWD) tools may be provided in a drill string, which tools include sensors to detect, for example, bore inclination. A transducer in the MWD tool generates a series or cycle of fluid-flow restrictions in the bore of the tool, representative of the sensed inclination of the bore. The restrictions create corresponding pressure pulses in the drilling fluid above the tool. The pressure pulses are detected and analysed on surface, to determine the measured condition.
Problems can be encountered when drilling if lost circulation drilling conditions are encountered; this is when a significant volume of drilling fluid is lost into permeable formations downhole. Thus, the volume of drilling fluid returning to surface is less than that pumped down the bore and on occasion drilling is performed with no returns, that is all the fluid pumped downhole is lost.
A further problem associated with lost circulation occurs when the drilling fluid pumps are stopped; the fluid level in the annulus drops quickly as fluid is lost into the permeable formation and the level of fluid within the drill pipe also drops to equalise the fluid level (known as the U-tube effect). This can create additional difficulties for the operation of MWD tools in such wells as, when the drilling fluid pumps are started again, the drill pipe must be filled with fluid before the MWD tool will start operating and sending signals to surface. If the MWD tool starts operating before the pipe refills the signal is likely to be lost in the air gap. Also, MWD tools can be damaged if they operate in the presence of a mixture of drilling fluid and gas.